The invention relates to a method of controlling the operation of a friction-based torque-transmitting system such as, in particular, a transmission or a continuously variable transmission in the drive train of a motor vehicle.
Systems for transmitting torque in the drive train of a motor vehicle by means of a frictional engagement, e.g., clutches (including lock-up clutches in the torque converters of automatic transmissions), continuously variable transmissions operating with endless flexible torque-transmitting devices, friction-drive mechanisms, as well as brakes, are increasingly being automated, i.e., operated under the control of appropriate actuators. The control or regulation of the torque to be transmitted through frictional engagement by the respective torque-transmitting system in each given case should be performed with the smallest amount of contact force possible in order to minimize the load on the actuators, to keep energy consumption small, and to achieve a high level of control/regulation accuracy. However, the amount of contact force should be sufficient to avoid a condition of permanent slippage that would rapidly wear out or destroy the friction-based torque-transmitting system. To avoid the risk of wear-out or destruction, an excess amount of contact pressure is used in most cases between elements transmitting torque through frictional engagement. Generally, an excess amount of contact pressure is also employed for the purpose of preventing unintended slippage due to wear of individual parts, settling effects, temperature effects or viscosity-based parameter changes.
One possibility to recognize a slippage condition between elements transmitting torque through frictional engagement is known from DE 195 44 061 A1. To recognize the limit between adhesion and slippage, a pressure-operated actuator element that controls the compressive contact force between the two elements transmitting torque through frictional engagement is supplied with an actuating pressure that includes a superimposed modulated signal. The signal variation is correlated, e.g., to the rate of rotation of an output shaft of the transmission. The limit between adhesion and slippage is determined as the point where the level of correlation falls below or rises above a predetermined value. The process of adding a modulated signal to the actuating pressure requires additional control elements and is relatively expensive.